Programming to mitigate memory cell performance differences

ABSTRACT

Methods for programming and memory devices are disclosed. In one such method for programming, a first programming voltage applied to control gates of a group of memory cells generates a maximum threshold voltage of the group of memory cell threshold voltages. A voltage difference between the maximum threshold voltage and a maximum target voltage is used as a gate step voltage for a second programming voltage. Fast and slow programming memory cells are determined from the distribution resulting from the second programming voltage. An effective gate voltage applied to the control gates of the fast programming memory cells is less than an effective gate voltage applied to the control gates of the slow programming memory cells during the third programming voltage.

TECHNICAL FIELD

The present embodiments relate generally to memory and a particular embodiment relates to non-volatile memory.

BACKGROUND

Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, flash drives, digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.

Non-volatile memory cells can be programmed as single level cells (SLC) or multiple level cells (MLC). Single level cells have two states, erased and programmed, that can typically be illustrated by the SLC distributions of FIG. 1A. Multiple level cells can typically be illustrated by the MLC distributions of FIG. 1B.

FIG. 1A shows that an erased state 100 of SLC memory, typically a logical 1 state, is shown as a negative state. The programmed state 101, typically a logical 0 state, is shown as a positive state.

FIG. 1B shows that the erased state 103 of MLC memory, typically a logical 11 state, is shown as the negative state. The programmed states 104-106 are illustrated as positive states that can be labeled as a logical 10 state 104, a logical 00 state 105, and a logical 01 state 106.

As is evident from FIGS. 1A and 1B, the non-volatile memory cells can be programmed from the erased state to one of the programmed states by increasing a threshold voltage (V_(t)) of a target memory cell from a negative threshold voltage to a target threshold voltage of one of the desired programmed states. The threshold voltage is typically increased by a series of incrementally increasing programming pulses, each programming pulse followed by a verify pulse to determine the present threshold voltage of the memory cell. The programming pulses start at an initial programming voltage and increase by a step voltage until the memory cell verifies as being programmed to the target threshold voltage.

Due to manufacturing and material variations in the integrated circuit die, memory cells of a memory array program at different rates. Thus, there will be slow programming memory cells and fast programming memory cells in any memory array. A programming method could end up over-programming the faster programming memory cells.

Depending on the threshold voltage to which a memory cell is being programmed, a total of five to eight programming/verify pulses might be used to properly program an SLC memory cell. Even though an entire page of memory cells is typically programmed at one time, the time required to program the large number of pages in a memory device can reduce the performance of a memory system during a programming operation.

For the reasons stated above, and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a way to improve programming performance of memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show typical prior art threshold voltage distributions for SLC and MLC memory.

FIG. 2 shows a schematic diagram of one embodiment of a portion of a NAND architecture non-volatile memory array.

FIG. 3 shows a dynamic gate voltage stage of one embodiment of a method for programming.

FIG. 4 shows a bit line biasing stage of one embodiment of the method for programming.

FIG. 5 shows an optional clean-up stage of one embodiment of the method for programming.

FIG. 6 shows a block diagram of one embodiment of a memory system that can incorporate the method for programming in accordance with FIGS. 3-5.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments. In the drawings, like numerals describe substantially similar components throughout the several views. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense.

FIG. 2 illustrates a schematic diagram of one embodiment of a portion of a NAND architecture non-volatile memory array 201 comprising series strings of non-volatile memory cells that can be programmed using the embodiments of the subsequently described method for programming. The present embodiments are not limited to the illustrated NAND architecture. Alternate embodiments can use NOR or other architectures as well.

The memory array 201 comprises an array of non-volatile memory cells 201 (e.g., floating gate) arranged in columns such as series strings 204, 205. Each of the cells are coupled drain to source in each series string 204, 205. An access line (e.g. word line) WL0-WL31 that spans across multiple series strings 204, 205 is coupled to the control gates of each memory cell in a row in order to bias the control gates of the memory cells in the row. Data lines, such as even/odd bit lines BL_E, BL_O, are coupled to the drains of the top memory cells of the series strings and eventually coupled to sense circuitry that detect the state of each cell by sensing a current or voltage on a selected bit line.

Each series string 204, 205 of memory cells is coupled to a source line 206 by a source select gate transistor 216, 217 and to an individual bit line BL_E, BL_O by a drain select gate transistor 212, 213. The source select gate transistors 216, 217 are controlled by a source select gate control line SG(S) 218 coupled to their control gates. The drain select gate transistors 212, 213 are controlled by a drain select gate control line SG(D) 214.

Each memory cell can be programmed as a single level cell (SLC) or a multiple level cell (MLC). Each cell's threshold voltage (V_(t)) is indicative of the data that is stored in the cell. For example, in an SLC, a V_(t) of 1.5V might indicate a programmed cell while a V_(t) of −0.5V might indicate an erased cell. The MLC uses multiple V_(t) ranges that each indicates a different state. Multiple level cells can take advantage of the analog nature of a flash cell by assigning a bit pattern to a specific V_(t) range. This technology enables the storage of data values representing two or more bits per cell, depending on the quantity of V_(t) ranges assigned to the cell.

One programming method that mitigates programming differences between slower and faster programming memory cells utilizes a two stage approach. An initial programming stage moves the threshold voltage quickly through the voltage difference between the erased state and the target threshold voltage. A second programming stage moves the threshold voltage quickly through the distribution resulting from the first stage. An optional stage can be used to finish the programming by increasing the threshold voltages of those memory cells that have not yet reached their target threshold voltages.

FIG. 3 illustrates a dynamic gate voltage stage of one embodiment of a programming method. This stage determines the dynamic gate voltage that is to be applied to the target memory cell control gates by the subsequent programming pulse.

The distribution illustrated is the result of an initial programming pulse, at a word line start voltage, being applied to a selected word line, or other grouping of memory cells, that is coupled to the control gates of the row of memory cells to be programmed. In one embodiment, the word line comprises a page or other grouping of memory cells to be programmed.

The word line start voltage can be determined, in one embodiment, in response to a difference between the erased voltage and the target voltage. For example, in an MLC device, if a memory cell is being programmed from the erased state to the highest programmed state, a larger word line start voltage is used. If the memory cell in the MLC device is only being programmed to the lowest programmed state, a lower word line start voltage can be used.

The bit lines that are coupled to the memory cells being programmed are initially biased at a low voltage (e.g., 0V). In one embodiment, only alternate bit lines are programmed at one time. For example, memory cells coupled to the even bit lines might be programmed initially and memory cells coupled to the odd bit lines could then be programmed.

The lower the bit line voltage, the faster the programming of the memory cells coupled to that particular bit line. Conversely, the higher the bit line voltage, the slower the programming of the memory cells coupled to that particular bit line. This is a result of at least the effective gate voltage that results from a difference between the programming pulse voltage applied to the memory cell control gate and the channel voltage caused by the bit line biasing. Thus, memory cells coupled to a bit line biased at an inhibit voltage (e.g., a supply voltage) would not program even when their respective control gates were biased with a programming voltage.

The initial pulse results in a Gaussian distribution of threshold voltages of memory cells 301 having a maximum threshold voltage (V_(tmax)) and a minimum threshold voltage (V_(tmin)). The Gaussian distribution is a result of the memory cells having their threshold voltages increased at different rates by the initial programming pulse. For example, the lower end of the distribution 301 includes the slower programming memory cells while the upper end of the distribution 301 includes the faster programming memory cells.

In order to detect the amplitude of the maximum V_(t), a series of read operations 302 are performed. Each read operation 1-n comprises a read pulse at a different voltage. If the read pulse turns on the target memory cell, as detected by the sense circuitry, the threshold voltage for that particular memory cell is less than or equal to that particular read voltage. The four different read operations 1-n at the four different voltages, as shown in FIG. 3, are for purposes of illustration only since the actual quantity of read operations depends on the quantity of read operations necessary to determine the maximum V_(t).

Once the maximum V_(t) of the initial distribution 301 is determined, it is possible to determine the largest approximate dynamic gate step voltage that can be used for increasing the threshold voltage of the target memory cell without affecting the width of the final target distribution 305. The final target distribution 305 is the distribution of threshold voltages of the target state for the group of memory cells being programmed.

In one embodiment, the voltage difference between the maximum target threshold voltage (V_(targetmax)) and V_(tmax) is used as the dynamic gate step voltage for the subsequent programming pulse. This voltage should not result in over-programming since it is less than V_(targetmax). The dynamic gate step voltage is added to the initial word line start voltage to obtain the voltage for the subsequent programming pulse.

FIG. 4 illustrates a bit line biasing stage of one embodiment of a method for programming. This stage determines the bit line biasing to be used for the subsequent programming pulse.

The distribution illustrated in FIG. 4 is the result of a second programming pulse being applied to the selected word line that is coupled to the control gates of memory cells to be programmed. The voltage of the second programming pulse is determined by the sum of the dynamic gate step voltage and the voltage of the initial programming pulse (e.g., word line start voltage). The resulting Gaussian distribution of threshold voltages of the memory cells 401 still has a maximum threshold voltage (V_(tmax)) and a minimum threshold voltage (V_(tmin)) but is shifted upward by approximately the dynamic gate step voltage that was used in the second programming pulse.

In the stage illustrated in FIG. 4, the distribution 401 is split into “slow” and “fast” programming cells by performing a verify operation with a pre-program verify (PPV) pulse at V_(PPV). This pulse determines which memory cells within the distribution are above V_(PPV) and which are below V_(PPV). V_(PPV), in one embodiment, is the mid-point voltage of the distribution. Alternate embodiments might use other voltages.

The memory cells above V_(PPV) are the “fast” programming memory cells and the bit line to which these memory cells are coupled is biased at the “fast” bit line bias voltage. The memory cells below V_(PPV) are the “slow” programming memory cells and the bit line to which these memory cells are coupled is biased at the “slow” bit line bias voltage. The “fast” bit line bias voltage is greater than the “slow” bit line bias voltage in order to slow down the programming of the faster memory cells. In one embodiment, the “fast” bit line bias voltage is 1.5V and the “slow” bit line bias voltage is 0V. Alternate embodiments might use other voltages within a range of 0V to V_(CC) to achieve substantially the same result of slowing down the fast programming memory cells. Once the bit line bias voltages are determined, those bit lines are locked at their respective voltages so that no further pre-programming verify operations are performed.

The different bit line bias voltages reduce the effective gate step voltage that is added to the previous programming voltage and applied to the word line of memory cells that are being programmed. As discussed previously, biasing the bit lines at the higher voltage causes the memory cell channel to be coupled up to that voltage so that the difference between the gate voltage and the channel voltage is reduced. Thus, the effective gate voltage is reduced for the “fast” programming memory cells as compared to the “slow” programming memory cells. For example, the previous two programming pulses might have resulted in a 3.8V effective step voltage for all of the memory cells being programmed. When bit lines coupled to memory cells that are programming faster than others are biased at 1.5V, the effective gate step voltage for the faster programming memory cells might be 1.8V while the effective gate step voltage for the slower programming memory cells might still be 3.8V. The bit line biasing stage tightens the final target distribution by simultaneously moving the faster programming memory cells by a smaller step and the slower programming memory cells by a larger step so that the resulting threshold voltage distribution is reduced.

FIG. 5 illustrates a clean-up stage of one embodiment of a method for programming. This stage can be used if any memory cells to be programmed remain below the final target distribution 505.

The distribution illustrated in FIG. 5 is the result of the third programming pulse being applied to the selected word line that is coupled to the control gates of memory cells to be programmed while the bit lines are biased as discussed previously. The programming voltage of the third programming pulse uses a unique gate step size that is different from the dynamic gate step voltage of the second programming pulse. However, as previously described, the effective gate voltage for each memory cell might be different.

The resulting Gaussian distribution of memory cells 505 might include all the memory cells being programmed. If any of the memory cells are still not programmed to the target state, additional programming pulses can be applied to the word line that is coupled to the control gates of the memory cells still to be programmed. The bit lines coupled to the memory cells that are already programmed are biased at an inhibit voltage that prevents them from being programmed further while the remaining memory cells are still experiencing the programming pulses. In one embodiment, the inhibit voltage can be a supply voltage.

The final target distribution 505 of FIG. 5 can represent the target state to which the memory cells are being programmed. In an SLC embodiment, this can be a logical 0 state. In an MLC embodiment, this can be any one of the non-erased states (e.g., 10, 00, 01).

FIG. 6 illustrates a functional block diagram of a memory device 600. The memory device 600 is coupled to an external processor 610. The processor 610 may be a microprocessor or some other type of controller. The memory device 600 and the processor 610 form part of a memory system 620. The memory device 600 has been simplified to focus on features of the memory that are helpful in understanding the present embodiments.

The memory device 600 includes an array 630 of memory cells (e.g., floating gate memory cells) such as the array of FIG. 3. The memory array 630 is arranged in banks of word line rows and bit line columns. In one embodiment, the columns of the memory array 630 comprise series strings of memory cells.

Address buffer circuitry 640 is provided to latch address signals provided through I/O circuitry 660. Address signals are received and decoded by a row decoder 644 and a column decoder 646 to access the memory array 630. It will be appreciated by those skilled in the art with the benefit of the present description that the number of address input connections depends on the density and architecture of the memory array 630. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.

The memory device 600 reads data in the memory array 630 by sensing voltage or current changes in the memory array columns using sense amplifier circuitry 650. The sense amplifier circuitry 650, in one embodiment, is coupled to read and latch a row of data from the memory array 630. Data input and output buffer circuitry 660 is included for bidirectional data communication as well as the address communication over a plurality of data connections 662 with the controller 610. Write circuitry 655 is provided to write data to the memory array.

Memory control circuitry 670 decodes signals provided on control connections 672 from the processor 610. These signals are used to control the operations on the memory array 630, including data read, data write (program), and erase operations. The memory control circuitry 670 may be a state machine, a sequencer, or some other type of controller to generate the memory control signals. In one embodiment, the memory control circuitry 670 is configured to control the timing and generation of voltages for the embodiments of the above-described programming method.

The memory device illustrated in FIG. 6 has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.

CONCLUSION

In summary, one or more embodiments provide programming of non-volatile memory cells while mitigating the performance differences between fast and slow programming cells. The programming method can also reduce the time for programming by reducing the quantity of programming pulses used to reach the target state.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. 

1. A method for programming a memory device, the method comprising: applying a first programming voltage to control gates of a group of memory cells; determining a step voltage responsive to a maximum target voltage and a maximum threshold voltage of the group of memory cells after application of the first programming voltage; applying a second programming voltage to the control gates of the group of memory cells wherein the second programming voltage is equal to a sum of the first programming voltage and the step voltage; and applying a third programming voltage to the control gates of the group of memory cells wherein an effective gate voltage of at least some of the memory cells is adjusted responsive to a programming rate of that particular memory cell.
 2. The method of claim 1 and further comprising applying additional programming voltages to control gates of memory cells that are not programmed after application of the third programming voltage.
 3. The method of claim 1 wherein the programming rate of a particular memory cell is determined responsive to a pre-program verify operation after the second programming voltage.
 4. The method of claim 1 wherein the first, second, and third programming voltages are applied to access lines coupled to the control gates of the group of memory cells.
 5. The method of claim 1 wherein the effective gate voltage is substantially a difference between a memory cell channel voltage and a gate voltage.
 6. The method of claim 1 wherein the step voltage is a dynamic step voltage that changes for subsequent first programming voltages responsive to different maximum threshold voltages.
 7. A method for programming a memory device, the method comprising: applying a first programming pulse to control gates of a row of memory cells; determining a maximum threshold voltage of the row of memory cells responsive to the first programming pulse; applying a second programming pulse to the control gates of the row of memory cells wherein a voltage of the second programming pulse is determined responsive to the maximum threshold voltage and a maximum target voltage; determining a first subgroup of memory cells of the row of memory cells that program faster than a second subgroup of memory cells of the group of memory cells; and applying a third programming pulse to the control gates of the row of memory cells while applying a voltage to data lines coupled to the first subgroup of memory cells that reduces an effective gate voltage of the first subgroup of memory cells as compared to an effective gate voltage of the second subgroup of memory cells.
 8. The method of claim 7 wherein determining the maximum threshold voltage comprises performing a plurality of read operations on the row of memory cells.
 9. The method of claim 7 wherein programming the row of memory cells comprises programming a page of memory cells.
 10. The method of claim 7 wherein the first programming pulse is applied to an access line coupled to the control gates of the row of memory cells.
 11. The method of claim 7 wherein applying the second programming pulse includes determining a step voltage to be added to an initial voltage of the first programming pulse by determining a difference between the maximum target voltage and the maximum threshold voltage.
 12. The method of claim 7 wherein the maximum target voltage is a maximum voltage of a distribution of threshold voltages of a target state.
 13. The method of claim 7 wherein, while applying the third programming pulse, the voltage applied to data lines coupled to the second subgroup of memory cells is lower than the voltage applied to data lines coupled to the first subgroup of memory cells.
 14. A memory device comprising: an array of memory cells; and memory control circuitry coupled to the memory array wherein the memory control circuitry is configured to control a programming operation of a group of memory cells, wherein the memory control circuitry is configured to control generation of a first programming pulse that is applied to control gates of the group of memory cells, determine a step voltage responsive to a resulting maximum threshold voltage of the group of memory cells, control generation of a second programming pulse having a voltage that is substantially equal to a sum of a voltage of the first programming pulse and the step voltage, and control an effective gate voltage on each memory cell of the group of memory cells responsive to a programming rate of each particular memory cell.
 15. The memory device of claim 14 wherein the array of memory cells comprises one of a NAND architecture or a NOR architecture.
 16. The memory device of claim 14 wherein the memory control circuitry is further configured to perform a pre-program verify operation to determine the effective gate voltage for each memory cell in the group of memory cell.
 17. The memory device of claim 16 wherein the pre-program verify operation comprises a pre-program verify pulse having a voltage within a distribution of threshold voltages generated responsive to the second programming pulse.
 18. The memory device of claim 17 wherein the memory control circuitry is further configured to determine that memory cells having a threshold voltage in the distribution of threshold voltages greater than the pre-program verify pulse voltage are fast programming memory cells.
 19. The memory device of claim 18 wherein the memory control circuitry is further configured to generate a lower effective gate voltage for the fast programming memory cells than for memory cells that program at a programming rate slower than the fast programming memory cells.
 20. The memory device of claim 17 wherein the memory control circuitry is further configured to determine that memory cells having a threshold voltage in the distribution of threshold voltages less then the pre-program verify pulse voltage are slow programming memory cells.
 21. The memory device of claim 14 wherein the memory control circuitry is further configured to generate a series of read operations, each read operation having a greater read voltage than a previous read voltage in the series of read operations, to determine the maximum threshold voltage of the group of memory cells.
 22. The memory device of claim 14 wherein the memory control circuitry is further configured to inhibit programming of memory cells having threshold voltages in a final target distribution while continuing to program memory cells not having threshold voltages in the final target distribution.
 23. The memory device of claim 22 wherein the memory cells comprise multiple level memory cells and the final target distribution is one of the states of a plurality of non-erased states.
 24. The memory device of claim 14 wherein the memory control circuitry is further configured to control generation of a voltage on each data line of the memory array such that the effective gate voltage for a particular memory cell coupled to a particular data line is increased if the particular memory cell is a fast programming memory cell.
 25. A method for programming a memory device, the method comprising: applying a first programming voltage to control gates of a group of memory cells; determining a step voltage responsive to a maximum target voltage and a maximum threshold voltage of the group of memory cells after application of the first programming voltage; applying a second programming voltage to the control gates of the group of memory cells, wherein the second programming voltage is equal to a sum of the first programming voltage and the step voltage; and applying a third programming voltage to the control gates of the group of memory cells wherein an effective gate voltage of at least some of the memory cells is adjusted responsive to a programming rate of that particular memory cell.
 26. The method of claim 25 wherein the step voltage is less than a maximum target voltage. 